1. Field of the Invention
This invention relates to magnetic recording mediums such as, for example, magnetic tapes for audio, video and computer service and floppy discs.
2. Description of the Prior Art
In the field of magnetic recording, there is a strong tendency towards high density recording and magnetic recording in a small size and large capacitance. To this end, extensive studies have been made on magnetic powders.
One such study is directed to fine division of magnetic powders. The use of such finely divided powder in a magnetic recording layer has been proposed as being effective in reduction of noises, improvement of a signal-to-noise ratio, and high density recording.
However, several problems are involved in the use of finely divided powders. One problem is that ordinary tape recorders have a tape end detector mechanism in order to detect a tape end of a magnetic recording tape and stop the tape on detection. More particularly, magnetic recording tapes have generally an optically transparent leader tape portion at opposite ends thereof and a magnetic recording layer whose transmission with respect to light having a wavelength of 8000 to 9000 angstrom is low. An optical sensor is used to detect the difference of the light transmission between the magnetic recording layer and the tape end and confirm the tape end portion. However, when finely divided magnetic powder is used, the light transmission of the resulting tape becomes high and thus, malfunction is apt to occur on detection by the tape end detector mechanism.
For solving the malfunction problem of the detector mechanism, there has been proposed a magnetic recording medium which uses large amounts of carbon black incorporated in a magnetic layer. Alternatively, the magnetic recording layer has been made thick. Both recording mediums are intended to decrease the light transmission of the recording layer which is increased by the fine division of magnetic powder.
However, the addition of carbon black in large amounts will bring about a lowering of magnetic characteristics because of the poor dispersability of carbon black in a binder resin. On the other hand, when the layer thickness is increased, the total thickness of the magnetic recording medium becomes large. To keep the total thickness at a given level in this condition, the thickness of a base has to be smaller. This is disadvantageous in that the if the medium is in the form of a tape, the breaking strength and travellability of the tape become poor. It will be noted that if the thickness of the magnetic recording layer is increased while keeping the thickness of a base as is ordinarily used, the total thickness of the magnetic medium increases as much. This inevitably requires a larger space for accomodating the medium. However, this is not practically possible since recording and reproducing apparatus and related devices are standardized and thus, the length of the medium must unfavorably be shortened.
Moreover, magnetic mediums are brought into frictional contact with magnetic heads when subjected to recording and reproducing operations and particularly at the time of freezing a frame. Accordingly, the magnetic layer has to be highly durable.